An Introduction to S-parameters

Whilst reading through this document you may find it useful to have access to the Polar Si9000e PCB Transmission Line Field Solver, as this will aid your understanding of S-parameters. Evaluation licenses are available from our offices, contact details can be found on slide 43.

S-parameters are a useful method for representing a circuit as a “black box”

S-parameters are a useful method for representing a circuit as a “black box” The external behaviour of this black box can be predicted without any regard for the contents of the black box.

S-parameters are a useful method for representing a circuit as a “black box” The external behaviour of this black box can be predicted without any regard for the contents of the black box. This black box could contain anything: a resistor, a transmission line or an integrated circuit.

A “black box” or network may have any number of ports. This diagram shows a simple network with just 2 ports.

A “black box” or network may have any number of ports. This diagram shows a simple network with just 2 ports. Note : A port is a terminal pair of lines.

S-parameters are measured by sending a single frequency signal into the network or “black box” and detecting what waves exit from each port. Power, voltage and current can be considered to be in the form of waves travelling in both directions.

S-parameters are measured by sending a single frequency signal into the network or “black box” and detecting what waves exit from each port. Power, voltage and current can be considered to be in the form of waves travelling in both directions. For a wave incident on Port 1, some part of this signal reflects back out of that port and some portion of the signal exits other ports.

I have seen S-parameters described as S11, S21, etc. Can you explain? First lets look at S11. S11 refers to the signal reflected at Port 1 for the signal incident at Port 1.

I have seen S-parameters described as S11, S21, etc. Can you explain? First lets look at S11. S11 refers to the signal reflected at Port 1 for the signal incident at Port 1. Scattering parameter S11 is the ratio of the two waves b1/a1.

I have seen S-parameters described as S11, S21, etc. Can you explain? Now lets look at S21. S21 refers to the signal exiting at Port 2 for the signal incident at Port 1. Scattering parameter S21 is the ratio of the two waves b2/a1.

S21? Surely that should be S12?? I have seen S-parameters described as S11, S21, etc. Can you explain? Now lets look at S21. S21 refers to the signal exiting at Port 2 for the signal incident at Port 1. Scattering parameter S21 is the ratio of the two waves b2/a1. S21? Surely that should be S12??

I have seen S-parameters described as S11, S21, etc. Can you explain? Now lets look at S21. S21 refers to the signal exiting at Port 2 for the signal incident at Port 1. Scattering parameter S21 is the ratio of the two waves b2/a1. S21? Surely that should be S12?? S21 is correct! S-parameter convention always refers to the responding port first!

I have seen S-parameters described as S11, S21, etc. Can you explain? A linear network can be characterised by a set of simultaneous equations describing the exiting waves from each port in terms of incident waves.   S11 = b1 / a1 S12 = b1 / a2 S21 = b2 / a1 S22 = b2 / a2 Note again how the subscript follows the parameters in the ratio (S11=b1/a1, etc...)

S-parameters are complex (i. e S-parameters are complex (i.e. they have magnitude and angle) because both the magnitude and phase of the input signal are changed by the network. (This is why they are sometimes referred to as complex scattering parameters).

These four S-parameters actually contain eight separate numbers: the real and imaginary parts (or the modulus and the phase angle) of each of the four complex scattering parameters.

Quite often we refer to the magnitude only as it is of the most interest. How much gain (or loss) you get is usually more important than how much the signal has been phase shifted.

What do S-parameters depend on? S-parameters depend upon the network and the characteristic impedances of the source and load used to measure it, and the frequency measured at. i.e. if the network is changed, the S-parameters change. if the frequency is changed, the S-parameters change. if the load impedance is changed, the S-parameters change. if the source impedance is changed, the S-parameters change.

What do S-parameters depend on? S-parameters depend upon the network and the characteristic impedances of the source and load used to measure it, and the frequency measured at. i.e. if the network is changed, the S-parameters change. if the frequency is changed, the S-parameters change. if the load impedance is changed, the S-parameters change. if the source impedance is changed, the S-parameters change. In the Si9000e the source and load impedance default is 50 Ohms, although this is configurable.

What do S-parameters depend on? S-parameters depend upon the network and the characteristic impedances of the source and load used to measure it, and the frequency measured at. i.e. if the network is changed, the S-parameters change. if the frequency is changed, the S-parameters change. if the load impedance is changed, the S-parameters change. if the source impedance is changed, the S-parameters change.

A little math… This is the matrix algebraic representation of 2 port S-parameters: Some matrices are symmetrical. A symmetrical matrix has symmetry about the leading diagonal.

A little math… This is the matrix algebraic representation of 2 port S-parameters: Some matrices are symmetrical. A symmetrical matrix has symmetry about the leading diagonal. In the case of a 2-port network, that means that S21 = S12 and interchanging the input and output ports does not change the transmission properties.

A little math… This is the matrix algebraic representation of 2 port S-parameters: Some matrices are symmetrical. A symmetrical matrix has symmetry about the leading diagonal. In the case of a symmetrical 2-port network, that means that S21 = S12 and interchanging the input and output ports does not change the transmission properties. A transmission line is an example of a symmetrical 2-port network.

A little math… Parameters along the leading diagonal, S11 & S22, of the S-matrix are referred to as reflection coefficients because they refer to the reflection occurring at one port only.

A little math… Parameters along the leading diagonal, S11 & S22, of the S-matrix are referred to as reflection coefficients because they refer to the reflection occurring at one port only. Off-diagonal S-parameters, S12, S21, are referred to as transmission coefficients because they refer to what happens from one port to another.

Larger networks: A Network may have any number of ports.  

Larger networks: A Network may have any number of ports.   The S-matrix for an n-port network contains n2 coefficients (S-parameters), each one representing a possible input-output path.

Larger networks: A Network may have any number of ports.   The S-matrix for an n-port network contains n2 coefficients (S-parameters), each one representing a possible input-output path. The number of rows and columns in an S-parameters matrix is equal to the number of ports.

Larger networks: A Network may have any number of ports.   The S-matrix for an n-port network contains n2 coefficients (S-parameters), each one representing a possible input-output path. The number of rows and columns in an S-parameters matrix is equal to the number of ports. For the S-parameter subscripts “ij”, “j” is the port that is excited (the input port) and “i” is the output port.

Yes i for output j for input — logical ;-) Larger networks: Yes i for output j for input — logical ;-) A Network may have any number of ports.   The S-matrix for an n-port network contains n2 coefficients (S-parameters), each one representing a possible input-output path. The number of rows and columns in an S-parameters matrix is equal to the number of ports. For the S-parameter subscripts “ij”, “j” is the port that is excited (the input port) and “i” is the output port.

Larger networks: A Network may have any number of ports.   The S-matrix for an n-port network contains n2 coefficients (S-parameters), each one representing a possible input-output path. The number of rows and columns in an S-parameters matrix is equal to the number of ports. For the S-parameter subscripts “ij”, “j” is the port that is excited (the input port) and “i” is the output port.

Sum up… S-parameters are a powerful way to describe an electrical network S-parameters change with frequency / load impedance / source impedance / network S11 is the reflection coefficient S21 describes the forward transmission coefficient (responding port 1st!) S-parameters have both magnitude and phase information Sometimes the gain (or loss) is more important than the phase shift and the phase information may be ignored S-parameters may describe large and complex networks If you would like to learn more please consider evaluating the Polar Si9000e PCB Transmission Line Field Solver. A 5 step process to generate, plot and export S-parameters is included in the following slides.

Using Polar Si9000e Field Solver to generate, plot and export S-parameters Step 1: Select the transmission line structure and enter the parameters

Using Polar Si9000e Field Solver to generate, plot and export S-parameters Step 2: Switch to the Frequency Dependent tab and enter the frequency range

Using Polar Si9000e Field Solver to generate, plot and export S-parameters Step 3: Select Calculate to generate S-parameters and plot graph

Using Polar Si9000e Field Solver to generate, plot and export S-parameters Step 4: The S-parameter data may be plotted in a number of useful ways

Using Polar Si9000e Field Solver to generate, plot and export S-parameters Step 5: The same S-parameter data may be exported to a number of formats Touchstone Excel .CSV

Further reading: Agilent papers http://www.sss-mag.com/pdf/an-95-1.pdf http://www.sss-mag.com/pdf/AN154.pdf   National Instruments paper http://zone.ni.com/devzone/nidzgloss.nsf/webmain/D2C4FA88321195FE8625686B00542EDB?OpenDocument Other links: http://www.sss-mag.com http://www.microwaves101.com/index.cfm http://www.reed-electronics.com/tmworld/article/CA187307.html http://en.wikipedia.org/wiki/S-parameters Online lecture OLL-140 Intro to S-parameters - Eric Bogatin Online lecture OLL-141 S11 & Smith charts - Eric Bogatin www.bethesignal.com